Electrochemical determination of copper in seawater based on a patch-type integrated electrode modified with gold nanoparticle-decorated carbon nanoflowers

Abstract

As both an essential trace metal nutrient and a heavy metal contaminant, copper (Cu) plays a significant yet dual role in marine ecosystems. Accurate determination of the labile fraction within dissolved Cu in the complex seawater matrix remains a major challenge. In this work, a novel electrochemical sensor based on a patch-type integrated electrode (P-tIE) modified with carbon nanoflowers (CNFs) and gold nanoparticles (AuNPs) was prepared for the determination of labile Cu in seawater. The three-dimensional flower-like structure of CNFs afforded a large specific surface area for loading AuNPs, which showed excellent electrocatalytic performance toward the voltammetric determination of Cu. Based on the synergistic effects of CNFs and AuNPs, the P-tIE sensor exhibited enhanced performance for Cu determination, exhibiting a linear range of 0.7–10 000 nM and a detection limit of 0.21 nM. Furthermore, the sensor was successfully applied to the direct determination of labile Cu in real seawater samples, providing a reliable tool for studies on the biogeochemical cycling of Cu in the ocean.

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Introduction

Copper (Cu) is one of the essential and indispensable metal elements for living organisms, playing a crucial role in biological systems.¹ In marine ecosystems, it is particularly vital for maintaining the normal growth of algae.^2,3 However, the growth and functionality of algae require an appropriate amount of Cu, as excessive Cu can cause severe damage to organisms, giving it dual characteristics of bioavailability and toxicity.⁴ Moreover, Cu in marine environments can be readily absorbed by organisms and subsequently amplified through the food chain, ultimately posing a threat to human health.⁵ In coastal waters, the biological availability and toxicity of Cu are not solely determined by its total dissolved concentration but are more profoundly influenced by its chemical speciation.⁶ Compared to total dissolved Cu (Cu_TD), the labile Cu (Cu_L), which includes free Cu ions, inorganically bound Cu, and weakly organically bound Cu, provides a more accurate estimate of the ecological risk posed by Cu.⁷ This is because Cu_L exerts the highest biological impact, directly participating in the physiological processes of plankton.⁸ Therefore, establishing accurate and reliable methods for the analysis of Cu concentration and speciation in seawater is crucial for in-depth research on the impact of Cu on marine environments and human health.⁹

The concentration of heavy metal ions can be measured through various techniques, such as atomic absorption spectrometry (AAS),¹⁰ atomic emission spectroscopy (AES),¹¹ and inductively coupled plasma-mass spectrometry (ICP-MS).¹² However, these methods are expensive, require high maintenance costs, involve complex sample pretreatment requirements, and have low sensitivity, making them unsuitable for the analysis of trace metals and the determination of concentrations.¹³ Electrochemical analysis methods have become a significant research direction in the field of heavy metal detection due to their notable advantages, including simple operation, rapid response, high detection sensitivity, ease of instrument miniaturization, and the capability for metal speciation analysis.¹⁴ Anodic stripping voltammetry (ASV) exhibits excellent sensitivity and selectivity in detecting trace metals due to the presence of the accumulation step during the detection process.^15,16 Furthermore, it does not require sample pretreatment or chemical reagent addition, making it exactly suitable for the determination of Cu_L.¹⁷

In recent years, nanomaterials have shown great potential in the field of electrochemical sensing due to their unique physical and chemical properties,¹⁸ such as high conductivity, a large specific surface area, and excellent stability.¹⁹ Their excellent electronic conductivity and significantly increased active specific surface area provided new ideas for the construction of high-performance electrochemical sensors.²⁰ Carbon nanoflowers (CNFs), as two-dimensional nanomaterials, have garnered significant attention due to their high electrochemical activity, large specific surface area, and environmental friendliness.²¹ Additionally, they can enhance the electrode conductivity without the need for adhesives, thereby effectively amplifying electrochemical signals. Meanwhile, gold nanoparticles (AuNPs) have also shown significant value in the field of electroanalysis.²² Their ability to improve the mass transfer efficiency, provide a high effective specific surface area, and exhibit excellent catalytic performance has provided critical technical support for electrochemical determination.^23,24 Additionally, AuNPs have shown outstanding catalytic performance in the electrochemical determination of target analytes such as Cu²⁺.²⁵

The patch-type integrated glassy carbon electrode (P-tIE) features a thin single-piece glassy carbon disc with a diameter of 2 mm serving as the working electrode material, bound directly to a chip printed with a carbon electrode (counter electrode) and an Ag/AgCl electrode (reference electrode). Similar to screen printed electrodes, P-tIE has the advantage of easy use due to its small size and integration of a three-electrode system.²⁶ Meanwhile, P-tIE ensures high reproducibility of the electrodes by cutting pieces from an entire thin glassy carbon disc as the working electrode material. In addition, the circular hole on the electrode sheet forms a dam around the glassy carbon patch, which can effectively protect the modified materials on the electrode surface and improve its stability.

In this work, a functional electrochemical sensor was successfully developed by modifying P-tIE with porous CNFs and AuNPs. The unique structure of CNFs provides abundant binding sites for AuNPs. AuNPs can enhance electron transfer and exhibit excellent catalytic activity, significantly improving the sensitivity for Cu determination. The AuNPs/CNFs/P-tIE combines high sensitivity and stability, enabling precise determination of trace Cu in a complex seawater matrix. Furthermore, Cu_L and Cu_TD in seawater samples were successfully determined using the AuNPs/CNFs/P-tIE, demonstrating its practical applicability.

Experimental

Reagents and instruments

The stock solution of Cu²⁺ was supplied by the National Research Centre for Certified Reference Materials (Beijing, China), and separate standard solutions were prepared by diluting the stock solution. All analytical grade chemical reagents such as chloroauric acid (HAuCl₄·4H₂O), ammonium fluoride (K₃[Fe(CN)₆]), potassium chloride (KCl), N,N-dimethylformamide (DMF), H₂O₂ and HNO₃ were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China) and used without further purification. CNFs were provided by Nanjing XFNANO Materials Tech. Co., Ltd, China. P-tIE was provided by Guangzhou Yuxin Sensing Technology Co., Ltd, China. Deionized water (18.2 MΩ cm specific resistance) was used throughout the experiment. Acetic acid buffer solution at pH 4.5 was prepared from 0.1 M HAc and 0.1 M NaAc.

All electrochemical experiments were carried out by using a CHI660E Electrochemical Workstation (CH Instruments, Shanghai, China) in connection with a patch-type integrated electrode. Scanning electron microscopy (SEM, Hitachi S-4800 microscope, Japan), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were adopted for the characterization of P-tIE, AuNPs/P-tIE, CNFs/P-tIE and AuNPs/CNFs/P-tIE. Energy dispersive X-ray spectroscopy (EDS, HORIBAEX-350, Japan) was used only for the elemental analysis. Inductively coupled plasma-mass spectrometry (ICP-MS, ELAN DRCII, PerkinElmer Instruments, USA) was used for comparative testing.

Construction of the AuNPs/CNFs/P-tIE

Scheme 1 illustrates the construction process of the AuNPs/CNFs/P-tIE for detecting Cu²⁺ in coastal waters, which begins with preparing the electrode surface. Specifically, CV was conducted in 0.5 M H₂SO₄ to remove impurities and activate the P-tIE surface, with parameters set as a scan rate of 50 mV s⁻¹, an initial potential of 0.2 V, and a final potential of 1.4 V. After carefully cleaning with deionized water and drying with the assistance of high-purity N₂, the prepared CNFs were uniformly dispersed in DMF to create a 2 mg mL⁻¹ dispersion. The ultrasonication process was carried out at 200 W for 120 min to ensure complete dispersion. Subsequently, 3 µL of this dispersion was deposited onto the electrode surface and allowed to air-dry at room temperature, resulting in the formation of CNFs/P-tIE. Following this, electrochemical deposition was performed using a 2 mM HAuCl₄ solution at a deposition potential of −0.3 V for a duration of 180 s, leading to the formation of AuNPs/CNFs/P-tIE.

Scheme 1 Schematic diagram of the construction of the AuNPs/CNFs/P-tIE.

Electrochemical analysis procedure

The CV characterization of P-tIE, CNFs/P-tIE, AuNPs/P-tIE and AuNPs/CNFs/P-tIE was conducted in the supporting electrolyte of 0.1 M HAc–NaAc buffer solution (pH 4.5) within the potential range of 0–1.2 V. For EIS characterization, the Nyquist plots of different electrodes were obtained in 0.1 M KCl solution containing 5 mM K₃[Fe(CN)₆] with the frequency ranging from 10⁻² to 10⁵ Hz, the amplitude of 0.005 V, and the potential of 0.20 V.

The electrochemical signal of Cu²⁺ was determined using ASV, which comprises two main processes: the accumulation process and the stripping process. The accumulation potential and time employed in this process were −0.2 V and 180 s, respectively. Following the accumulation phase, an equilibration period of 10 s was allowed, after which a potential scan was conducted from −0.4 V to 0.3 V using square wave voltammetry (SWV) to obtain the stripping current response signal of Cu²⁺. The parameters for the SWV scan included an initial potential of −0.4 V, a final potential of 0.3 V, an amplitude of 0.025 V, a potential increment of 0.004 V, a frequency of 10 Hz, and a quiet time of 2 s. After one measurement, a constant potential of 0.3 V and 180 s was applied to conduct the electrode cleaning. All ASV measurements were conducted under strictly controlled static conditions: a defined volume of electrolyte was directly dispensed onto the surface of patch-type integrated electrodes (P-tIEs), with no agitation or vibration applied during deposition.

Preparation of real seawater samples

Seawater samples were collected from the Yellow River estuary and Yantai Sishili Bay (Northern Yellow Sea, China). The polypropylene containers used were previously soaked in nitric acid (HNO₃, 5%, 24 h) and thoroughly cleaned with deionized water. After collecting, the samples for the Cu_L determination were filtered through a 0.45 μm cellulose acetate filter membrane and stored in a refrigerator at 4 °C. For Cu_TD determination, 10 µL of 30% hydrogen peroxide and 10 µL of HNO₃ were added to 10 mL of seawater sample and then digested with a 500 W UV lamp (Metrohm MVA-UV 705, Switzerland) in an acid-washed quartz tube for 30 minutes. The samples were stored at 4 °C until analysis.

Results and discussion

Characterization of the AuNPs/CNFs/P-tIE

The surface morphology of different modified electrodes was investigated by SEM. SEM images of the CNFs/P-tIE are shown in Fig. 1A and B. A flower-like appearance with a three-dimensional structure can be observed clearly. The high-magnification SEM image in Fig. 1B reveals the fine flower-like structural details of the CNFs. The diameter of CNFs is approximately 600 nm. As for AuNPs/CNFs/P-tIE, nanoparticles with a diameter of approximately 80–200 nm were uniformly loaded on the CNFs (Fig. 1C). To verify that the loaded nanoparticles are AuNPs, EDS was conducted (Fig. 1D). According to the characteristic peaks of gold appearing at about 2.14 and 9.70 keV, it could be concluded that the nanoparticles were exactly AuNPs.

Fig. 1 SEM images of the CNFs/P-tIE (A and B) and AuNPs/CNFs/P-tIE (C), and the EDS pattern of AuNPs/CNFs/P-tIE (D).

To confirm that AuNPs were uniformly loaded on the surface of CNFs, elemental mapping analysis of AuNPs/CNFs/P-tIE was conducted (Fig. 2). Fig. 2A displays the overall structure of the AuNPs/CNFs used for elemental mapping analysis, and Fig. 2B, C and D show the distributions of elements C, Au and O, respectively. It can be seen that except for C and O elements, Au element (from AuNPs) was uniformly distributed on the nanomaterial.

Fig. 2 Elemental mapping images of the AuNPs/CNFs/P-tIE (A) and the corresponding SEM images for C (B), Au (C) and O (D) elements.

The CV characterization of different P-tIEs was conducted in the supporting electrolyte of 0.1 M HAc–NaAc buffer solution (pH 4.5) within the potential range of 0 to 1.2 V. Fig. 3A presents the CV curves of P-tIE, CNFs/P-tIE, AuNPs/P-tIE, and AuNPs/CNFs/P-tIE at a scan rate of 50 mV s⁻¹. It can be seen that no distinct redox peaks are present in the CV curve of P-tIE. After the modification of CNFs, the current response is significantly enhanced, which may be attributed to the increase in electrode specific surface area and electron transfer rate. The CV curve of AuNPs/P-tIE displays characteristic oxidation and reduction peaks of Au⁰ at 0.9 V and 0.5 V, respectively. Interestingly, in the CV curve of AuNPs/CNFs/P-tIE, the intensity of the characteristic redox peaks of AuNPs exhibits a certain degree of enhancement due to the loading of AuNPs on the CNF surface. It can be concluded that CNFs can provide a significant number of active sites for loading AuNPs.

Fig. 3 (A) CV curves of the bare P-tIE, CNFs/P-tIE, AuNPs/P-tIE and AuNPs/CNFs/P-tIE in acetate buffer solution (0.1 M, pH 4.5). (B) Nyquist plots of the bare P-tIE, CNFs/P-tIE, AuNPs/P-tIE and AuNPs/CNFs/P-tIE in 0.1 M KCl solution containing 5 mM K₃[Fe(CN)₆].

To verify the electron transfer ability of different electrodes, as well as the synergistic effects of AuNPs and CNFs, EIS was conducted. Fig. 3B shows the Nyquist plots of the P-tIE, AuNPs/P-tIE, CNFs/P-tIE and AuNPs/CNFs/P-tIE measured in 5 mM K₃[Fe(CN)₆] solution with 0.1 M KCl. The charge transfer resistance (R_ct) of the electrode active surface corresponds to the semicircle diameter in the high-frequency region, with a larger diameter indicating an increased R_ct. It can be observed that the bare P-tIE has the largest R_ct, followed by CNFs/P-tIE, AuNPs/P-tIE, and AuNPs/CNFs/P-tIE. It is no doubt that both CNFs and AuNPs can improve the electron transfer ability at the electrode/solution interface. Furthermore, the R_ct of AuNPs/CNFs/P-tIE is significantly lower than the other three electrodes, suggesting the synergistic effects of AuNPs and CNFs.

Electrochemical response of AuNPs/CNFs/P-tIE to Cu²⁺

The SWV response curves of P-tIE, AuNPs/P-tIE and AuNPs/CNFs/P-tIE in acetate buffer solution containing 500 nM Cu²⁺ are shown in Fig. 4. The experimental results show that the bare P-tIE has almost no current response to 500 nM Cu²⁺, whereas the AuNPs/P-tIE exhibits an obvious oxidation current signal at about −0.1 V. It is obvious that the enhanced current response is caused by the excellent electrocatalytic ability of AuNPs in the voltammetric determination of Cu²⁺. Particularly noteworthy is that the current response of AuNPs/CNFs/P-tIE is further significantly higher than that of AuNPs/P-tIE, which is mainly attributed to the unique structure of CNFs. The three-dimensional flower-like structure of CNFs afforded a large specific surface area and abundant active sites for loading AuNPs, which greatly enhances the electrocatalytic performance. In addition, CNFs exhibit excellent chemical stability, which improves the reliability and reproducibility of the AuNPs/CNFs/P-tIE for Cu determination.

Fig. 4 SWV curves of the bare P-tIE, AuNPs/P-tIE and AuNPs/CNFs/P-tIE for 500nM Cu²⁺ in acetate buffer solution (0.1 M, pH 4.5).

Optimization for Cu²⁺ determination with the AuNPs/CNFs/P-tIE

The effects of the experimental parameters of accumulation potential, accumulation time, drop coating volume of CNFs and electrodeposition time of AuNPs on the voltammetric response of Cu²⁺ on the AuNPs/CNFs/P-tIE were studied. The variation of peak current obtained with the AuNPs/CNFs/P-tIE for 500 nM Cu²⁺ was investigated when the accumulation potential was in the range of −0.6–0.1 V (Fig. 5A). The peak current increases continuously as the accumulation potential decreases from 0.1 V to −0.2 V and reaches a maximum at −0.2 V. This phenomenon can be attributed to the fact that the reduction accumulation efficiency of Cu²⁺ at the electrode surface increases significantly with a negative shift of the potential within this range. However, when the potential further negatively shifts from −0.2 V to −0.6 V, the peak current decreases instead. This may be due to the competitive reduction of dissolved oxygen at more negative potentials. Therefore, −0.2 V is adopted as the optimal accumulation potential, under which AuNPs/CNFs/P-tIE can achieve efficient accumulation of Cu²⁺.

Fig. 5 Effects of the accumulation potential (A), accumulation time (B), drop coating volume of CNFs (C) and electrodeposition time of AuNPs (D) on the current response of 500 nM Cu²⁺ obtained with the AuNPs/CNFs/P-tIE.

Then, the effect of accumulation time on electrode performance was further investigated and is shown in Fig. 5B. It can be seen that the peak current tends to increase significantly with the accumulation time from 30 s to 180 s. This phenomenon is mainly due to the fact that more Cu²⁺ is reduced and enriched on the electrode surface with the increase of accumulation time. The observed non-linearity in the relationship between signal intensity and deposition time primarily results from the progressive saturation of active sites on the AuNPs/CNF-modified electrode surface rather than irreproducible hydrodynamic conditions. As deposition proceeds, the gradual occupation of available binding sites causes the response to deviate from initial linearity, with stabilization occurring beyond 180 s due to complete surface coverage. Considering that a longer accumulation time can decrease the determination efficiency, 180 s is used in this work.

Fig. 5C shows the effect of the drop coating volume of CNFs. The oxidation peak current shows a significant upward trend as the drop coating volume of CNFs increases from 1.0 μL to 4.0 μL. CNFs can provide a large specific surface area and enhance the accumulation ability of Cu²⁺ on the electrode surface. In addition, with the increase of CNFs, more AuNPs can be loaded on the electrode surface. When the drop coating volume exceeds 3.0 μL, the current response shows a decreasing trend. The excessive thickness of the CNF layer may hinder the electron transport between AuNPs and the electrode surface. Based on this, 3.0 μL was used as the optimal drop coating volume of CNFs.

Electrodeposition time of AuNPs is another critical parameter influencing the voltammetric determination performance of the electrode. To evaluate its effect, the electrodeposition time of AuNPs was varied from 30 to 210 s, as shown in Fig. 5D. The peak current increases gradually with the deposition time from 30 to 180 s due to the progressive loading of AuNPs on the CNF surface, which effectively increases the electroactive surface area. However, beyond 180 s, the current response reaches a plateau as the available nucleation sites become fully occupied. Further extension of deposition time may lead to AuNP agglomeration without enhancing the catalytic activity, which accounts for the observed plateau in current response at deposition times beyond 180 s. The optimal deposition time of 180 s represents a crucial balance that maximizes the electrode's active surface area while preventing saturation. This optimized AuNP/CNF configuration, with its high density of well-dispersed nanoparticles, enables the efficient accumulation and detection of Cu²⁺ across the wide linear range observed in the subsequent analytical measurements, as the surface remains unsaturated during the copper detection step. Thus, 180 s was selected as the electrodeposition time of AuNPs for the subsequent experiments.

Performance evaluation of the AuNPs/CNFs/P-tIE for Cu²⁺ determination

Fig. 6A and B show the SWV plots and corresponding calibration curves of AuNPs/CNFs/P-tIE for Cu²⁺ concentrations ranging from 0.7 to 10 000 nM under the above optimal experimental conditions. It can be seen that the obtained oxidation peak current I_p has a linear relationship with the Cu²⁺ concentration in the range of 0.7–10 nM and 10 to 10 000 nM, with the regression equations I_p = 4.59C + 132.72 (R² = 0.993) and I_p = 0.41C + 270.71 (R² = 0.996), respectively. This biphasic linear response represents a characteristic feature of electrochemical sensing platforms, wherein the enhanced sensitivity at lower analyte concentrations arises from kinetically controlled deposition at preferential active sites, whereas the attenuated sensitivity at elevated concentrations signifies a transition to diffusion-limited processes governed by mass transport constraints. The limit of detection (LOD) of the AuNPs/CNFs/P-tIE for Cu determination under the optimal conditions was calculated as 0.21 nM (S/N = 3). The performance comparison of AuNPs/CNFs/P-tIE with other previously reported electrodes for Cu²⁺ determination and applicable waters is shown in Table 1. It can be seen that the AuNPs/CNFs/P-tIE exhibits excellent performance in terms of linear range and sensitivity.

Fig. 6 SWV curves (A) and the corresponding calibration curves (B) obtained on the AuNPs/CNFs/P-tIE with successive addition of 0.7 nM, 1 nM, 2 nM, 5 nM, 7 nM, 10 nM, 20 nM, 50 nM, 70 nM, 100 nM, 200 nM, 500 nM, 700 nM, 1000 nM, 2000 nM, 5000 nM and 10 000 nM Cu²⁺ (from bottom to top) in acetate buffer (pH 4.5) solution.

Table 1 Comparison of the analytical performance for Cu²⁺ and applicable waters of the AuNPs/CNFs/P-tIE with that of previously reported modified electrodes

Electrodes^a	Methods^b	Linear range (nM)	LOD (nM)	Applicable waters	Ref.
a Electrodes: PANI – polyaniline; GCE – glassy carbon electrode; H-D/G-E – hybrid diamond/graphite electrode; BiFE – bismuth film-modified electrode; NSRG – N/S co-doped graphene; G-IrNS – LGL-protected iridium needle electrode modified with gold nanoparticles; Bi-GC/RDE – glassy carbon rotating disk electrode modified with a bismuth film; ZIF-67 – a cobalt-based zeolitic imidazole framework; RGO – reduced graphene oxide. b Methods: SWASV – square wave anodic stripping voltammetry; DPASV – differential pulse anodic stripping voltammetry.
HxTiS2 nanosheet-PANI/GCE	SWASV	25–5000	0.7	River water	27
H-D/G-E	DPASV	157.5–15 750	88.1	Tap water	28
Au@MnO2/GCE	DPASV	20–1000	5.0	Seawater	29
BiFE	DPASV	78.7–1731	14.8	River water	30
NSRG/GCE	SWASV	5–3000	6.0	Seawater	31
Au microband	SWASV	157.5–1575	12.76	River water	32
G-IrNS	SWASV	3.1–157.4	0.78	Seawater	33
Bi-GC/RDE	SWASV	0.629–5.51	10.5	River water	34
Fe-ZIF-67/RGO/Nafion/GCE	SWASV	10–1000	6.54	Tap water	35
AuNPs/CNFs/P-tIE	SWASV	0.7–10 000	0.21	Seawater	This work

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Reproducibility, repeatability and selectivity

To evaluate the performance of the AuNPs/CNFs/P-tIE for copper determination in complex seawater samples, its reproducibility, repeatability and selectivity were systematically investigated. The repeatability of the AuNPs/CNFs/P-tIE for Cu determination was evaluated through 20 consecutive measurements of 50 nM Cu²⁺ in acetate buffer (pH 4.5) using the same electrode (Fig. S1A). The relative standard deviation (RSD) of the obtained stripping peak current was 2.07%. The reproducibility of the AuNPs/CNFs/P-tIE was investigated in 50 nM Cu²⁺ using six independently prepared AuNPs/CNFs/P-tIEs with the same method, and the RSD of the stripping peak current was 2.52% (Fig. S1B). The results revealed that the prepared AuNPs/CNFs/P-tIE had excellent repeatability and reproducibility when used for Cu determination. Furthermore, in order to evaluate the anti-interference ability of the electrode, a series of foreign ions were added into the acetate buffer (pH 4.5) solution with 50 nM Cu²⁺ (Fig. S1C). The results showed that 5 μM (100-fold of Cu²⁺ concentration) SO₄²⁻, Ca²⁺, Mg²⁺, K⁺ and Na⁺, 2.5 μM (50-fold of Cu²⁺ concentration) Cd²⁺, Zn²⁺ and Ni²⁺ and 0.5 μM (10-fold of Cu²⁺ concentration) Bi³⁺ and Pb²⁺ will not affect the determination of Cu²⁺ (<5% of current change). So, the prepared AuNPs/CNFs/P-tIE also had an excellent selectivity for the voltammetric determination of Cu.

Analysis of real seawater samples

The seawater samples were quantified by the standard addition method. Before determination, in order to verify the accuracy of the AuNPs/CNFs/P-tIE for Cu determination in seawater, the results obtained by using the sensor for Cu_TD were compared with those obtained by ICP-MS (Table S1). The consistency of the results obtained through these two methods indicates that the prepared AuNPs/CNFs/P-tIE has reliable accuracy. Fig. 7A shows the typical SWV curves obtained on the AuNPs/CNFs/P-tIE in a seawater sample with the successive addition of 0, 20, 40 and 60 nM Cu²⁺ and the corresponding linear regression curve. The concentration of Cu in this seawater sample was calculated to be 10.66 nM. The linear titration curves obtained for Cu_L indicated minimal interference from strong ligands under our experimental conditions. This can be attributed to the rapid ASV measurement performed after each standard addition, which limited the time available for complexation reactions with strong ligands. Furthermore, the relatively high concentration of the added Cu²⁺ standard, combined with the low concentration of strong unsaturated ligands in the coastal waters, resulted in effective saturation of the available binding sites, thereby preserving the linearity of the response without causing significant changes in the Cu_L peak current. The determination results of Cu_L and Cu_TD in six seawater samples are shown in Fig. 7B. It can be seen that the Cu_L and Cu_TD concentrations of these seawater samples range from 4.43–14.86 nM and 18.59–71.02 nM, respectively. The proportion of Cu_L to Cu_TD ranges from 20.92% to 48.76%. Notably, these variations in the Cu_L/Cu_TD ratio are primarily governed by dynamics in terrestrial inputs and dissolved organic matter (DOM) content. Specifically, DOM can reduce the Cu_L ratio through complexation, while salinity variations indirectly regulate copper speciation by influencing the ionic strength and the complexation capacity of inorganic ligands.

Fig. 7 (A) SWV curves and the corresponding linear regression curve obtained with the AuNPs/CNFs/P-tIE in the seawater sample with successive addition of 0 nM, 20 nM, 40 nM and 60 nM Cu²⁺ (from bottom to top). (B) Concentrations of Cu_L and Cu_TD in seawater samples obtained with the AuNPs/CNFs/P-tIE and the contribution proportions of Cu_L to Cu_TD.

Conclusions

In summary, a patch-type integrated electrochemical sensor was successfully developed using three-dimensional CNFs decorated with AuNPs for voltammetric determination of different Cu species in seawater. The synergistic effect of CNFs and AuNPs endows the sensor with enhanced analytical performance. CNFs provide a large number of active sites for loading AuNPs which exhibit excellent electrocatalytic performance. Meanwhile, they also improve the specific surface area of the electrode significantly. The practical determination of Cu_L and Cu_TD in seawater samples using the AuNPs/CNFs/P-tIE shows its applicability for real seawater monitoring. Furthermore, the developed AuNPs/CNFs/P-tIE platform shows promising potential as a versatile sensing strategy for detecting various heavy metal contaminants in diverse aquatic environments. Future research will explore its capability for simultaneous multi-analyte detection, thereby providing new technological support for comprehensive water quality monitoring.

Author contributions

Shengjie Chu: conceptualization, methodology, writing – original draft, formal analysis, and data curation. Fei Pan: validation and writing – review & editing. Yuxuan Zhang: supervision and writing review & editing. Haitao Han: writing – review & editing, conceptualization, supervision, and validation. Dawei Pan: project administration, funding acquisition, methodology and resources. Xueping Hu: methodology and resources.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: reproducibility, repeatability and selectivity of AuNPs/CNFs/P-tIE and determination results of Cu_TD in real seawater samples obtained with the AuNPs/CNFs/P-tIE and by ICP-MS. See DOI: https://doi.org/10.1039/d5an01083g.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2021YFD190090201), the National Natural Science Foundation of China (42207522), the Natural Science Foundation of Shandong province (ZR2024QB330), the Taishan Scholar Project of Shandong Province (tsqn202103133) and the Special Fund for the Scholar Program of Yantai City.

References

1. M. Achary, S. Panigrahi, K. Satpathy, R. Prabhu and R. Panigrahy, Reg. Stud. Mar. Sci., 2016, 6, 96–108.
2. M. Lohan and A. Tagliabue, Elements, 2018, 14, 385–390.
3. M. Heller and P. Croot, Mar. Chem., 2015, 173, 253–268.
4. H. Biswas and D. Bandyopadhyay, Mar. Environ. Res., 2017, 131, 19–31.
5. L. Kong, Microorganisms, 2022, 10, 1853.
6. M. Sinoir, E. Butler, A. Bowie, M. Mongin, P. Nesterenko and C. Hassler, Mar. Freshwater Res., 2012, 63, 644–657.
7. E. Barber-Lluch, M. Nieto-Cid, J. Santos-Echeandía and P. Sanchez-Marín, Sci. Total Environ., 2023, 901, 165989.
8. H. Wang, V. Ebenezer and J. Ki, J. Microbiol., 2018, 56, 426–434.
9. J. A. Facey, S. C. Apte and S. M. Mitrovic, Toxins, 2019, 11, 643.
10. D. Rajendiran, N. Harikrishnan and K. Veeramuthu, Sci. Rep., 2025, 15, 9161.
11. L. Perring and M. Basic-Dvorzak, Anal. Bioanal. Chem., 2002, 374, 235–243.
12. C. Lau, X. Lu, K. Hoy, T. Davydiuk, J. Graydon, M. Reichert and X. Le, J. Environ. Sci., 2025, 153, 302–315.
13. S. John, S. Olukotun, T. Kupi and M. Mathuthu, Appl. Water Sci., 2024, 14, 202.
14. B. Zhang, J. Chen, H. Zhu, T. Yang, M. Zou, M. Zhang and M. Du, Electrochim. Acta, 2016, 196, 422–430.
15. T. Skiba, N. Medvedev, A. Tsygankova and Y. Panasenko, Anal. Lett., 2024, 57, 237–249.
16. H. Han, D. Pan, S. Zhang, C. Wang, X. Hu, Y. Wang and F. Pan, Bull. Environ. Contam. Toxicol., 2018, 101, 486–493.
17. D. Crmaric, S. Marcinek, A. Cindric, D. Omanovic and E. Bura-Nakic, Mar. Chem., 2025, 268, 104471.
18. L. Zhang, M. Yin, J. Qiu, T. Qiu, Y. Chen, S. Qi, X. Wei, X. Tian and D. Xu, Biosens. Bioelectron.:X, 2022, 10, 100102.
19. C. Luo, X. Liu, F. Liu, N. He, R. Yu and X. Liu, J. Electroanal. Chem., 2022, 904, 115930.
20. T. Nguyen, V. Cao, T. Pham and T. Le, Electroanalysis, 2019, 31, 2538–2545.
21. Q. Zhang, S. Ma, L. Zhang, H. Wang, X. Zhuo, G. Wang, Y. Shen, K. Zhang and S. Su, Sens. Actuators, B, 2023, 374, 132819.
22. M. Amjadi and Z. Abolghasemi-Fakhri, Sens. Actuators, B, 2018, 257, 629–634.
23. T. Yamazaki, T. Ikeda, B. Lim, K. Okumura, M. Ishida and K. Sawada, J. Sens., 2011, 2011, 190284.
24. A. Afkhami, M. Soltani-Shahrivar, H. Ghaedi and T. Madrakian, Electroanalysis, 2016, 28, 296–303.
25. H. Han, J. Wang, D. Pan, Y. Li and C. Wang, Environ. Pollut., 2023, 316, 120687.
26. M. Luis-Sunga, S. Carinelli, G. García, J. L. González-Mora and P. A. Salazar-Carballo, Sensors, 2024, 24, 2570.
27. X. Gan, H. Zhao, S. Chen, H. Yu and X. Quan, Anal. Chem., 2015, 87, 5605–5613.
28. Y. Guo, N. Huang, B. Yang, C. Wang, H. Zhuang, Q. Tian, Z. Zhai, L. Liu and X. Jiang, Sens. Actuators, B, 2016, 231, 194–202.
29. H. Wei, D. Pan, X. Hu, M. Liu, H. Han and D. Shen, Microchim. Acta, 2018, 185, 258.
30. N. M. Thanh, N. Van Hop, N. D. Luyen, N. H. Phong and T. T. Tam Toan, Adv. Mater. Sci. Eng., 2019, 2019, 1826148.
31. X. Tan, Z. Li, X. Wang, M. Xu, M. Yang and J. Zhao, New J. Chem., 2022, 46, 10618–10627.
32. B. Patella, T. Narayan, B. O'Sullivan, R. Daly, C. Zanca, P. Lovera, R. Inguanta and A. O'Riordan, Electrochim. Acta, 2023, 439, 141668.
33. Y. Li, H. Han, C. Wang, Y. Liang, D. Pan and H. Wang, Chemosphere, 2023, 313, 137366.
34. A. V. Bounegru, L. P. Georgescu, C. Iticescu and C. Apetrei, J. Electroanal. Chem., 2025, 979, 118924.
35. R. Zhu, L. Ding, Q. Li, H. Dong, M. Shi, Z. Song, S. Li, H. Chen and J. Zhang, Results Chem., 2025, 15, 102266.

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